View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Shinshu University Institutional Repository Burrow Plasticity in the Deep-Sea Isopod Bathynomus doederleini (Crustacea: Isopoda: Cirolanidae) Author(s) :Toshinori Matsui, Tohru Moriyama and Ryuichi Kato Source: Zoological Science, 28(12):863-868. 2011. Published By: Zoological Society of Japan DOI: http://dx.doi.org/10.2108/zsj.28.863 URL: http://www.bioone.org/doi/full/10.2108/zsj.28.863 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. ZOOLOGICAL SCIENCE 28: 863–868 (2011) ¤ 2011 Zoological Society of Japan Burrow Plasticity in the Deep-Sea Isopod Bathynomus doederleini (Crustacea: Isopoda: Cirolanidae) Toshinori Matsui1, Tohru Moriyama2* and Ryuichi Kato3 1Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan 2Young Researchers Empowerment Center, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan 3Graduate School of Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan We investigated whether the deep-sea isopod Bathynomus doederleini has the capacity to change burrow length in response to changes in environmental conditions. We observed burrowing behav- ior in individuals that were placed on substrates with either simple (ST) or complex (CT) surface topographies. Individuals in the ST group (N = 10) constructed seven burrows. The mean ratio of the burrow length to body length was 1.8. The individuals in the CT group (N = 10) constructed eight burrows with a mean ratio of burrow length to body length of 2.5. Thus the burrows were signifi- cantly longer in the CT group. In addition, the isopods in the CT group often incorporated a cham- ber in the mid-section of the burrow. Our results may be used to infer the determinants of burrow morphology and speculate about the lifestyle of this species in the deep sea. Key words: burrow, plasticity, deep sea, isopod, Crustacea, Bathynomus doederleini was a shelter, not a nest. More recently, Iwasaki et al. INTRODUCTION (2001) reported that burrow construction was initiated when- The mesopelagic zone (200–1,000 m below sea level) ever an individual made contact with a vertical acrylic wall, contains the greatest biomass and diversity of animal life in which was placed away from the lateral side of the aquarium the ocean (Warrant and Locket, 2004). The isopod on a gelatin substrate. In the absence of this wall, the indi- Bathynomus is distributed primarily within this zone, and is vidual dug along the side of the aquarium. These observa- classified as part of the littoral-bathyal benthos (Wolf, 1970). tions suggest that the burrowing behavior of B. doederleini This isopod is a highly mobile, voracious scavenger that is induced by mechanical stimulation of the head. Further- dominates the fauna associated with large dead animals more, these two studies suggest that the length of the bur- (Sekiguchi et al., 1981). row is equal to that of the body in both natural and artificial There has been an increasing awareness that highly substrates. mobile epibenthic invertebrates play a significant role in the In contrast, Matsuoka and Koike (1980) reported that dynamics of deep-sea benthic communities (Hessler et al., the burrow length of a Miocene period Bathynomus sp. was 1978). Thus, it is important to understand the biology of approximately six times longer than that of the occupant’s Bathynomus and its role in the deep benthic environment body length. In addition, the burrow had an expanded (Tsuo and Mok, 1991). Researchers have studied the clas- region, approximately twice as wide as the occupant’s body sification, geographical distribution, and development of width, in the mid-section of the burrow. Thus, it has Bathynomus (Holthuis and Mikulka, 1972; Sekiguchi et al., remained unclear whether there are differences in burrow 1982; Tsuo and Mok, 1991), but little is known about the length among captive and wild B. doederleini. ecology and behavior of this species because of the diffi- To address this, we attempted to duplicate features of culty observing animals in situ at such great depths. How- the B. doederleini natural habitat in the aquarium and eval- ever, Bathynomus is able to survive for several months in uate whether the morphology of burrows dug by extant B. aquaria at atmospheric pressure, affording researchers the doederleini resembled those dug in the Miocene. The opportunity to study its behavior. For example, Sekiguchi surface topography of the sea floor is highly complex, in part (1985) reported burrowing behavior in a captive Bathynomus due to the presence of carcasses, e.g., whale skeletons doederleini individual. Because the burrow was equal in (Gage & Tyler, 1991). Thus, we hypothesized that B. length to the isopod, the author concluded that the burrow doederleini construct longer burrows in topographically com- plex substrates when compared with simple substrates, * Corresponding author. Phone: +81-(0)268-21-5589; which are typically used in experiments with captive B. Fax : +81-(0)268-21-5884; doederleini. To test this, we documented burrowing behav- E-mail: [email protected] ior in B. doederleini that were placed on substrates with sim- doi:10.2108/zsj.28.863 ple or complex surface topography. 864 T. Matsui et al. of 100 cm from the surface of the substrate. The camera was con- MATERIALS AND METHODS nected to a digital high-resolution recorder (Sharp DV-ACW72). The Animals LEDs were turned on during the experiments to allow recording We collected specimens of B. doederleini using baited traps in while the aquaria were in the dark. Sagami Bay, Japan. The animals were then transferred to tanks We randomly selected 20 isopod individuals from the holding containing recirculated sea water (10 ± 0.2°C; pH 8 ± 0.2; 30 ± 1 tanks and divided them into two groups: ST (N = 10) and CT (N = ‰S) and held in the dark for six months. Each individual was fed 10). We measured the body length and width of each individual and slices of raw squid (5 g) once every two weeks. We placed the indi- determined their sex (Tsuo and Mok, 1991). Each isopod was indi- viduals in separate buckets while feeding. vidually placed on the center of the aquarium on the surface of the substrate (ST group) or in one of the tubes (CT group). We Experimental aquaria recorded the behavior of animals in both groups during a 6 h period. We used a transparent cylindrical aquarium (60 cm in diameter, At the end of the experiment, we measured the burrow length. 40 cm in height) that was placed in the laboratory (room tempera- If an individual was found in the burrow, it was gently removed by ture, 15°C). We created two types of substrate having differing sur- hand to avoid damage to the burrow. We injected green fluid into face topography, simple (ST) and complex (CT). For both groups, each empty burrow to create a cast. The burrow morphology was the aquaria were filled with 0.3% agar to a depth of 25 cm. The con- photographed using a digital camera (Nikon Coolpix P5100). centration of agar was determined during preliminary tests in which Each individual was used only once. The agar substrate and B. doederleini individuals were placed in aquaria filled with various seawater were changed between individuals. The plastic tubes concentrations agar substrata. We observed the behavior of the iso- were also washed with water between individuals. pods to ensure they were able to crawl and burrow in the agar. After the agar set, sea water was poured into the aquarium to a depth of Statistical analyses 10 cm above the surface of the agar substrate. The experimental We evaluated the differences in mean body length, mean body setup was left until the temperature at the center of the substrate width, mean time from the initial movement on the substrate to the reached 16 ± 1°C. completion of a burrow, and the mean time spent in a burrow In half of the aquaria, we placed five plastic, transparent T- between the two groups using an unpaired Student’s t-test. This test shaped tubes (6 cm diameter, 12 cm trunk length, 9 cm branch was also used to evaluate differences in the mean ratio of the bur- length) on the surface of the substrate (Fig. 1). The tubes were con- row length to the occupant’s body length between burrows with and nected to the base of the aquarium using thin plastic poles to pre- without chambers in the CT group. We evaluated the difference in vent movement. The distance between each tube and the aquarium the mean ratio of burrow length to the occupant’s body length wall or a neighboring tube was sufficient to allow passage of the iso- between the treatment groups using Welch’s t-test. We tested for pods. This more complex, maze-like substrate surface was intended normality of the data using a Shapiro-Wilk test.